'monotonic': Monotonic dominance constrains the function to require the
effect (slope) in the direction of the dominant dimension to be greater
than that of the weak dimension for any point in both lattice and linear
models. Both dominant and weak dimensions must be monotonic. The
constraint is guranteed to satisfy at the end of training for linear
models, but might not be strictly satisified for lattice models. In such
cases, increase the number of projection iterations.